Ellipsometry metrology involving apparatus in which samples to be studied are mechanically positionable and changes in reflected light from the samples is used in feature identification or characterization of the surface and bulk aspects of solid materials is becoming of increasing necessity as the dimensions being studied become smaller. The terminology "sample" is used for explanation simplicity. In metrology, the apparatus and analytical principles involved are useable in single, batch and on line types of operation in arrangements where an incident beam of polarized light is reflected from a sample under study and differences in the polarization of the reflected light that are produced by the reflection from the sample provide the measured quantities. The metrology operations require tedious data taking involving calibrated interaction of mechanical and optical components. One of the most influential parameters is the angle of incidence of the beam of polarized light, when the beam reflects from the sample. Any error in the value assumed in the data taking for the angle of incidence, especially where small dimensions are involved, is frequently only identified after considerable apparatus calibration time and effort. There are needs in the ellipsometry metrology art for ways of providing accurate assumed values for all parameters and in the case of the angle of incidence the need is particularly acute.